1. Field of Invention
The present invention relates in general to gamma ray spectroscopy well logging and, more particularly, to improved methods and apparatus for investigating the lithology of earth formations by neutron activation gamma ray spectroscopy.
2. The Prior Art
Heretofore, various procedures have been known for investigating the mineral composition, or lithology, of earth formations. In U.S. Pat. No. 3,566,117 to Tixier, one such technique is described in which porosity-dependent signals derived from a two-detector neutron porosity tool are cross plotted against bulk density-related signals from a two-detector gamma-gamma mudcake compensated density tool to provide an indication of lithology. Another method, described in U.S. Pat. No. 3,590,228 to Burke, combines measurements of sonic travel time, bulk density, and neutron-derived porosity to derive two lithology indices, known as M and N indices, which are then compared, e.g., by cross plotting, to identify the major lithological constituents of the formation. Several other methods are known to obtain lithology information from combinations of sonic, density, and neutron measurements. One such method, known as the Dual Mineral Method, uses a cross-plot of the neutron and density data to arrive at values of porosity and apparent matrix density of the formation. The sonic measurement is then used to indicate zones of secondary porosity and to help define the lithology. A more general method, which is based on the Dual Mineral Method, has been developed for lithology interpretations of formations having mixtures of silica, limestone, dolomite, anhydrite, and clay, or that are mixtures of any two specified minerals plus clay, and takes into account both formation shaliness and hydrocarbon effects. The above techniques are described in detail by Poupon et al. in "Log Analysis in Formations with Complex Technologies," Journal of Petroleum Technology, August, 1971, pp. 995-1005. These foregoing techniques, while generally providing an accurate indication of the formation lithology, all require data to be measured by at least two different types of logging tools.
Methods are known, however, which do not require such multiple tool data gathering operations. U.S. Pat. No. 4,055,763 to Antkiw, for example, discloses a method for using a gamma ray spectroscopy tool to identify the lithology of earth formations based on mesurements of the relative proportions of selected elements in the formation. For instance, by determining the relative contributions of silicon, calcium and iron to the capture gamma ray spectrum of the formation and then forming various ratios of the measured elemental contributions, it is possible to detect the presence of shales or determine whether the formation is limestone or sandstone. A generally similar method to that of Antkiw is described in U.S. Pat. No. 3,930,154 to Scott et al, wherein, following constituent analysis of the capture gamma ray energy spectrum for a formation of unknown lithology to obtain the relative constituent proportions, the volume fractions of the various constituents are calculated and ratios of certain volume fractions are formed to provide indications of lithology, including a dolomite/limestone indicator. Although these spectroscopy techniques afford useful lithology indicators, it is desirable to provide still further information regarding the lithological make-up of formations and, in particular, to provide an improved, straightforward spectroscopy technique for differentiating between dolomite and limestone formations and for separating such formations from other lithologies. It is also desirable to provide a dolomite/limestone indicator which is not affected by the presence of sandstone in the formation.